Gee, that's too bad. I assumed that the 1.6.4 Windows build was delayed
because it was a lower priority. Do you suppose this position was taken
because there are no developers wishing to keep it alive?

Well this is interesting. The linker can't find that because
MPI::Datatype::Free isn't implemented on the Windows build (in
datatype_inln.h). It's declared in datatype.h though. It's not there in
the Linux version either, so I don't know where the Linux build is getting
that symbol from, that link should fail too. Is the C++ version of OpenMPI
actually broken overall?

The Windows support is another issue. I think it's semi-officially
deprecated.